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Barring using Cocoa dialogs and integrating them into your script (way out of the scope of this question – and of my expertise, too), you can’t. AppleScript’s display dialog does not allow for typographic formatting (and neither does the only script friendly alternative to AppleScript’s inbuilt abilities, Carsten Blüm’s Pashua).

If all you need is for the first (supposedly most important) part of the text to be emphasized, you can use display alert instead of display dialog

display alert "this is the important part" message "… and this is not"